Correct Option
The correct option isOil and gas layers occur above the water layer.
Explanation
Hydrocarbons (petroleum and natural gas) accumulate in porous subterranean rock formations known as reservoirs. The physical arrangement of these substances relative to groundwater is governed by density differentiation and their immiscible nature (insolubility in water).
Option Analysis
- They are found mixed homogeneously with groundwater. is incorrect: Petroleum and natural gas are non-polar organic compounds, whereas water is polar. They are immiscible and do not form a homogeneous mixture; instead, they separate into distinct layers.
- Gas is found below the oil layer due to higher density. is incorrect: Natural gas is the least dense of the three fluids. Therefore, it always accumulates at the highest point in the trap, above the oil layer, not below it.
- Oil and gas layers occur above the water layer. is correct: The density of water is higher than that of oil, and oil is denser than gas. Consequently, in a reservoir trap, water forms the bottom layer, oil floats above the water, and gas occupies the space above the oil (Gas > Oil > Water).
- They occur in open pools on the ocean floor. is incorrect: Hydrocarbons are not found in open caverns or pools but are trapped within the microscopic pore spaces of sedimentary rocks (like sandstone or limestone) under high pressure.
Key Takeaway: In a subterranean reservoir, fluids stratify based on density: Gas (lightest) is at the top, Oil is in the middle, and Water (densest) is at the bottom.